Microfluidics is a powerful tool for chemical and biological manipulations and assays. Benefits of microfluidics include reduced reagent consumption and analysis time, as well as the ability to integrate multiple functions on a single device. Two basic families of microfluidic devices exist. The first family consists of channel microfluidic devices, in which fluids are manipulated as continuous flows in micron-dimension channels. The second family consists of droplet-based microfluidic (DMF) devices, in which a liquid is transported in the form of droplets across a planar surface or between two parallel surfaces, rather than as a continuous stream in a channel. In DMF devices, the sequence of droplet movements can be programmable, allowing the same device to be used to perform multiple different assays.
In DMF devices, voltages are sequentially applied to an electrode array to move a droplet across a planar surface to achieve such functions as droplet dispensing, droplet motion, droplet splitting, and droplet merging. However, microscopic and macroscopic irregularities in the planar surface and/or chemical residues left on the planar surface from the prior movement of the droplet or other droplets in the DMF device generate a hydrodynamic drag force that cannot be overcome by the motive force generated by an applied voltage less than the breakdown voltage between the electrodes. Conventional DMF devices overcome this problem by sandwiching the droplet between two plates having planar surfaces, and filling the gap between the surfaces of the plates with a background matrix of oil that reduces the hydrodynamic drag between the droplet and the surfaces of the plates. However, the use of an oil background matrix severely limits the usefulness and flexibility of the DMF device. For example, the oil forms an impenetrable barrier between the droplet and the substrate surface, making it impossible to perform surface chemistry. Moreover, the requirement that the droplets remain immiscible in the oil imposes a limitation on the chemical composition of the droplet.
Accordingly, what is needed is a way to overcome hydrodynamic drag in a DMF device without the limitations resulting from the use of a background matrix of oil.